Plant microfossil analysis

Starch grains

Our understanding of the uses and values of plants in the past is limited in comparison to other foodstuffs, and this is especially true in prehistory. This is in large part because of the poor preservation of macroscopic plant remains, the limited scope for bimolecular techniques to detect all but oily and waxy plant products. Plant microfossils, such as starches and phytoliths, often preserve better than other plant tissues and offer the best target for analysis.

Phytoliths are microscopic pieces of silica produced by plants during evapo-transpiration. They form in and around cells meaning that they take on shapes and

sizes characteristics of the cells they are associated with. This gives them taxonomic value for identification. Charred deposits offer havens for preserving plant microfossils like phytoliths due to their thermal durability (<1000°C). Phytoliths charred by cooking have also been found more resilient to destruction by pH extremes beyond <pH3 and >pH9, their normal range. The close association with cooking pots and with other organic traces of food within the charred deposit places their culinary interpretation beyond doubt.

Starch granules are semi-crystalline, water insoluble grains that can account for up to 25% of the total weight of a storage root, tuber crop or other underground storage organ. Starches are carbohydrate polymers deployed by plants into energy storage organs. Within the field of plant microfossil analysis, starches have long been known for their potential as an indicator of diet, as they display morphological variation that is taxonomically significant. This variation includes their shape and size characteristics, but also features such as the way the granules respond to different light conditions during microscopy. Granules exhibit birefringent properties (image, below) that can assist with plant identification.

The high temperatures to which carbonised deposits have been exposed can accelerate tissue degradation in plants making them more prone to subsequent organic degradation. However, the durability of many plant microfossils is attested by their recovery from heavily carbonised deposits that have also undergone mechanical processing in some instances.